Hitherto, the commonest procedure used in determining the moisture content of a substance or material is to hold same at a temperature above the boiling point of water until such time as all of the water present has evaporated. The difference in the weight of the substance or material before and after heating, represent the weight of the water lost; from which the moisture content can be calculated. During this heating procedure, the temperature of the material being examined cannot exceed the boiling point of water, until all of the water is evaporated. Thus, although the heating capacity must be substantially greater than merely the heat required to evaporate the water from the substance being examined, the temperature which is utilized usefully, is only marginally above the boiling point of water. Accordingly, drying ovens used for moisture determinations commonly are set at 105 degrees Celcius (compared to the boiling point of water at 100 degrees Celcius).
Another major reason, necessitating a minimum temperature in drying materials of biological origin, are the changes which occur at elevated temperatures. For example, case hardening of a material effectively traps the water inside, making moisture determinations impossible. In these instances, it may be necessary to reduce the temperature at which water boils by applying vacuum to the material. In still other instances, where even these lower temperatures are unacceptable, sublimation of water from the frozen sample by vacuum techniques may be necessary (i.e., freeze-drying, as it is commonly called). Although all these procedures based on the transfer of heat are well known, and can be highly automated, they require extended periods of time, usually 24 hours or longer.
One method of determining the moisture content of a material is disclosed in Canadian Pat. No. 729,885 issued Mar. 15, 1966 to Crown Zellenbach Canadian Limited. The patented method is limited to a thin porous material and wherein such material is passed through a drying chamber. It is obvious the entire material will be elevated in temperature because of being a thin porous material and because of being subjected to an enclosed heated chamber. Also because of using a drying chamber the process is highly energy intensive.
In many industrial processes where virtually instantaneous water content measurements are desired or required, it is common to measure an electrical property of the substance where a relationship between the electrical property and moisture content of the substance can be established. Moisture values estimated from electrical properties such as resistance, conductivity and capacitance are acceptably accurate, for the most part, when the materials on which measurements are being made are closely similar. However, it has been found that components of biological nature in particular, other than water, affect these electrical properties.
These components are primarily dissolved constituents and, as they vary in concentration and in relative proportions, the relationship between electrical properties and moisture values changes. Thus, these "instantaneous" types of measurement suffer from increasing inaccuracy, depending upon factors such as the nature of the biological material, its geographic origin, the climatic conditions under which it has developed and others.
For example, in the lumber industry electrical resistance meters are in common use for measuring moisture content. Notwithstanding that the meters have different calibrations for different wood species and corrections are applied for temperature differences, instances where actual values differ substantially from values predicted on the basis of electrical resistance measurement occur constantly. For unexplained reasons (since logs are not an annual crop), the relationship between electrical resistance values and actual moisture content of the lumber also appears to change with time, and must be re-evaluated periodically.
Another major shortcoming of these electrical techniques is that they require direct contact between the substance whose moisture content is being evaluated, such as wood, and the electrical device. For a resistance meter the pins, between which the resistance is measured, must be driven into the wood. This has obvious limitations for the on-line measurement of moisture values.
Another major shortcoming of the electrical techniques is their inability to measure what is called "free" water. In wood, electrical resistance increases with increasing water content up to about 30%, approaching the inherent value of water. Above this level, where the wood fibres are saturated with water, higher moisture levels cause no change in electrical resistance, and a resistance meter is effectively useless. Moisture levels in excess of 30% are common in unseasoned lumber, particularly for certain species. Radiofrequency methods of measuring high moisture content levels have been attempted, but as before, its accuracy has been found to be low. Under these circumstances, only oven drying techniques are accurate for obtaining meaningful high moisture measurements. The drawback of oven drying however is that it is energy intensive as the entire substance to be checked for moisture content is elevated in temperature.